Tae-Wook Koh

Optical Outcoupling Enhancement in Organic Light‐Emitting Diodes: Highly Conductive Polymer as a Low‐Index Layer on Microstructured ITO Electrodes

Organic light-emitting diodes (OLEDs) are now entering mainstream display markets and are also being explored for next-generation lighting applications. In both types of applications, high external quantum efficiency (EQE) is of premium importance for both low power consumption and long lifetime. It is well known that one of the bottlenecks in achieving high EQE in OLEDs is the low light-extraction efficiency, which is limited to <20%, mostly because total internal reflections occurring at interfaces between optically distinctive layers confine a significant portion of the light within the substrate (1⁄4 ‘‘substrate-confined’’ mode) or within the organic/indium tin oxide (ITO) layers (1⁄4 ‘‘wave-guided’’ mode). Hence, many device structures have been proposed to extract light that would not normally be outcoupled: some have attempted to extract the light that is confined in a substrate by introducing structures such as microlens array (MLA) or pyramidal arrays on the backside of the substrate, where other research groups have tried to extract the light that is confined within organic/ITO layers by introducing optical structures such as photonic crystals or low-index grids that can disrupt the wave-guiding of the light within the organic/ITO layers. The lattermay be carried out in a direct way by converting wave-guided modes directly into outcoupled modes or in an indirect way by converting wave-guided modes into substrate-confined modes and then extracting them with structures mentioned in the former approach. Criteria for choosing a specific method or structure over others depend on the target applications: for display applications, methodologies such as MLA and substrate structuring are often avoided due to their optical blurring effect; for lighting applications, such methods are readily accepted, but complex processes that add too much cost are generally not welcomed. In both cases, compatibility with a common fabrication technique and large-area fabrication is strongly preferred, and the Lambertian angular dependence and the absence of spectral dependence are also preferred in most situations. Here we introduce a novel anode structure based on micropatterned ITOs coated with high-conductivity (HC)-grade poly(3,4ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) layers. This proposed electrode structure can improve the outcoupling efficiency of OLEDs in a relatively simple way without severe spectral dependence, blurring (optional), or deviation from the normal angular dependence. Figure 1 illustrates a tilted top-view and cross-section of the proposed anode structure and its working principle. ITO layers are patterned so that the square opening (Wo Wo) repeats in a square lattice layout with a spatial period ofWt. For simplicity, we consider a situation where Wt1⁄4 2 Wo. In this case, the width of ITO strips (WITO (1⁄4Wt–Wo)) next to each opening equalsWo, and the ITO-less portion is 25% per each unit cell. The spatial period and the dimension of openings are chosen to be sufficiently larger than the emission wavelength so that a geometric optic approach can be valid and spectral dependence may be ignored. Each pattern may have a taper angle utaper, as defined in Figure 1a. A high-conductivity PEDOT:PSS (Baytron PH 500, HC Starck, Inc.) layer is coated throughout the anode area over the patterned anode. Organic layers and metal cathodes are then deposited to complete the device. Note that the light emitted with a small angle within the emission layer, which would normally be wave-guided throughout the organic/ ITO layers, is now guided either solely within organic layers (the ray in red coming from the left side in Figure 1b) or solely within ITO layers (the ray in blue coming from the right side in Figure 1b), because the refractive index ( 1.42 at l1⁄4 550 nm) of PEDOT:PSS is lower than those of ITOs and typical organic layers used in OLEDs. Upon hitting the structured region once or multiple times, the guided light will change its direction so that it can be directly coupled out. Some portion of the wave-guided mode can also be converted to a substrate-confined mode (see Figure S1 in Supporting Information). It has to be noted that patterned ITO electrodes alone without the PEDOT:PSS overcoat would not work effectively, because organic layers and ITO layers are optically almost identical due to their similar refractive indices. The low refractive index of a PEDOT:PSS layer is indeed the characteristic that enables internal reflections at the structured interfaces, which are among the key processes that must occur in order for the guided light to be converted to an outcoupled or a substrate-confined mode. In addition to the optical benefits noted above, it is also critical that, in the region without ITO, the HC-grade PEDOT:PSS layer provides an electrical sheet conduction and works as an anode independently so that there is no inactive area in the devices. In fact, we note each anode region consisting solely of PEDOT:PSS is surrounded by ITO electrodes, resembling the conductive grid structure suggested by Leo and his coworkers that can ensure

A Facile Route to Efficient, Low‐Cost Flexible Organic Light‐Emitting Diodes: Utilizing the High Refractive Index and Built‐In Scattering Properties of Industrial‐Grade PEN Substrates

An industrial-grade polyethylene naphthalate (PEN) substrate is explored as a simple, cost-effective platform for high-efficiency organic light-emitting diodes (OLEDs). Its high refractive index, combined with the built-in scattering properties inherent to the industrial-grade version, allows for a significant enhancement in outcoupling without any extra structuring or special optical elements. Flexible, color-stable OLEDs with efficiency close to 100 lm W(-1) are demonstrated.

Doping-Free Inverted Top-Emitting Organic Light-Emitting Diodes With High Power Efficiency and Near-Ideal Emission Characteristics

Inverted top-emitting organic light-emitting diodes (ITOLEDs) with high power efficiency and near-ideal emission characteristics are demonstrated by using the combination of the following: 1) an electron-injection layer composed of Cs2CO3, which lowers the turn-on voltage; 2) an electron-transporting layer with optimal electron mobility, which enhances the electron current and thus improves the carrier balance; and 3) a dielectric/metal/dielectric multilayer electrode that works as a damage-free top transparent anode optimized to achieve high efficiency and ideal emission characteristics. By this approach, ITOLEDs with power efficiency values of 3.8 and 30 lm W-1 are demonstrated in fluorescent and phosphorescent types, respectively, at a luminance value of 1000 cd m-2 with little distortion in spectral/angular characteristics.

Enhanced light extraction in organic light-emitting devices: Using conductive low-index layers and micropatterned indium tin oxide electrodes with optimal taper angle

We present our study on organic light-emitting diodes (OLEDs) in which outcoupling is enhanced based on a bilayer electrode consisting of a conductive low-index layer and micro-patterned indium tin oxide (ITO) layers. Optical simulation reveals that the taper angle of an ITO pattern is among the most critical parameters influencing the outcoupling efficiency in the proposed structure. A fabrication method based on a lift-off process is then employed to control the taper angle of the ITO pattern to be in the optimal range. OLEDs with the proposed electrode structure exhibit 50%–70% enhancement in external quantum efficiency over reference devices.

Enhanced and balanced efficiency of white bi-directional organic light-emitting diodes

We report on the characteristics of enhanced and balanced white-light emission from bi-directional organic light-emitting diodes (BiOLEDs) enabled by the introduction of micro-cavity effects. The insertion of an additional metal layer between the indium tin oxide anode and the hole transporting layer results in similar light output of our BiOLEDs in both top and bottom direction and in reduced distortion of the electroluminescence spectrum. Furthermore, we find that by utilizing MC effects, the overall current efficiency can be improved by 26.2% compared to that of a conventional device.

Towards highly efficient and highly transparent OLEDs: advanced considerations for emission zone coupled with capping layer design

Strategies to achieve efficient transparent organic light-emitting diodes (TrOLEDs) are presented. The emission zone position is carefully adjusted by monitoring the optical phase change upon reflection from the top electrode, which is significant when the thickness of the capping layer changes. With the proposed design strategy, external quantum efficiency and transmittance values as high as 15% and 80% are demonstrated simultaneously. The effect of surface plasmon polariton (SPP) loss from thin metal electrodes is also taken into account to correctly describe the full scaling behavior of the efficiency of TrOLEDs over key optical design parameters.

CCDC 981456: Experimental Crystal Structure Determination

Related Article: Hye Jin Bae, Jin Chung, Hyungjun Kim, Jihyun Park, Kang Mun Lee, Tae-Wook Koh, Yoon Sup Lee, Seunghyup Yoo, Youngkyu Do, and Min Hyung Lee|2014|Inorg.Chem.|53|128|doi:10.1021/ic401755m